Previous studies have shown that proteolytic degradation of glutamine synthetase (GS) in Escherichia coli involves two steps: First, a single histidine in each GS subunit is oxidized by a mixed-function oxidation reaction. Second, the modified enzyme is proteolyzed. The purpose of this project is to isolate a protease which preferentially degrades the modified GS. A protease that degrades modified GS ten times more rapidly than native GS has been partially purified from E. coli extracts. The proteolytic activity precipitates in a 60-70% ammonium sulfate cut, does not bind to DEAE ion exchanger at pH 7.5, and gel chromatographs with an apparent molecular weight of 110,000 Daltons. The protease activity is optimum at pH 9.5, is affected by Zn, Cu, and Mn, and is inhibited by aprotinin.